1. Field of the Invention
The present invention relates to a film forming, photosensitive, heat-resistant resin composition and a process for the formation of a heat-resistant insulating resin pattern using such composition. The film formed from the photosensitive, heat-resistant resin composition (in the present specification, the term "film" is used in a broad sense to indicate various coatings, films, thin films, etc.) adheres excellently to base materials and has good heat resistance, moisture resistance, insulating properties, toughness, abrasion resistance, and weather resistance, and further, can be formed at a low cost. Further, the process for formation of a pattern according to the present invention is advantageous in that the use of a photoresist for masking is not necessary, and a pattern of heat-resistant insulating resin film formed by this process is useful as one of the components of a circuit board or a semiconductor device, and can be utilized, for example, as a protective film or an insulating film (layer insulating film) in printed circuits, printed boards, wiring boards, and electronic components for high-density mounting. The term "base material" or "base material to be treated" as used in the present specification refers to mean general-use substrates including semiconductor substrates, ceramic substrates, metallic substrates, and various layered films and wiring.
The present invention also relates to a polymeric composite and a production process thereof. More particularly, the present invention relates to a polymeric composite or "polymer blend" having a particles-in-matrix microstructure. A matrix component of the polymeric composite comprises a polyimide resin which has a good heat resistance, and therefore can be advantageously utilized in the fields of electronics, transporting equipments, aircraft and aerospace industries, and a particles component thereof comprises the conventional curable resin, particularly an acrylic resin or a phosphazenic resin. Since it can exhibit an excellent heat resistance, insulating properties and toughness which are characteristic of the polyimide resins including polyimide engineering plastics, and also can show a low stress, the polymeric composite of the present invention can be advantageously used as a heat-resistant insulating layer or film such as a protective film or an interlayer insulator in printed circuits, printed boards and wiring boards and the like for high-density mounting as well as in electronic components or devices.
2. Description of the Related Art
Solder jointing is utilized for mounting IC chips in printed circuits, printed boards, wiring boards and electronic components including multi-chip modules or the like (as exemplified in FIGS. 1 and 2) for high-density mounting, and accordingly, the insulating films used for the above-described circuit substrates should be able to withstand the heat applied during the soldering. Further, since a large amount of information must be rapidly processed, reductions in the size and increases in the capacity of information processors are required. Semiconductor devices constituting a major part of such processors have been integrated through a reduction in the size of unit elements, thus enabling LSIs and VLSIs to be put to practical use. The quantity of heat (heat value) emitted from a semiconductor device increases with an increase in the integration of unit elements, and in an LSI, the heat value amounts to about 10 W.
Integration is achieved by a multi-layerization of the circuit, and when forming a semiconductor integrated circuit element, insulators are needed for layer insulation and surface protection, and inorganic insulators, such as phosphosilicate glass (abbreviated to "PSG"), silicon dioxide (SiO.sub.2) and silicon nitride (Si.sub.3 N.sub.4), are used for this purpose.
Although the above-described inorganic insulators have excellent properties such as dielectric strength and heat resistance, it is difficult to form an insulating film having a large thickness therefrom because inorganic insulating films are formed by a chemical vapor deposition process (abbreviated to "CVD").
The surfaces of semiconductor substrates to be coated with an insulating film have a significant number of fine uneven portions having a large aspect ratio, and inorganic insulating films take on a form which is similar to the form of the surface of the substrate, and this raises the problems of impossibility of smoothing the substrate surface and insufficient coverage.
Accordingly, investigations have been made into the use of polyimide, which can be coated by spin coating and has excellent heat resistance, not only as a surface protective film of a semiconductor integrated circuit element but also as a layer insulating film. A polyimide film for use as a surface protective film or a layer insulating film can be formed by dissolving a polyimide precursor, such as bismaleimide, polyamide acid or diamine, in a solvent, such as N-methyl-2-pyrrolidone (abbreviated to "NMP"), coating the resulting solution on a semiconductor substrate by a method such as spin coating, and heating the coating to a temperature of 150 to 400.degree. C. to cause a cyclodehydration reaction to occur, and thus cause the coating to be cured. The cyclodehydration reaction is also referred to as a polyimidalization reaction. Since the polyimide used herein has no photosensitivity in itself, a fine polyimide pattern may be formed by coating a photoresist on a polyimide precursor film, forming a resist pattern by photolithography, transferring the resist pattern to the lower layer by wet etching or plasma etching to form a pattern comprising a polyimide precursor, and heat-treating the pattern to cause a cyclodehydration reaction to occur, to thereby form a polyimide pattern.
A polyimide having photosensitivity in itself, which enables a polyimide to be patterned to any form, has been developed and is commercially available from various manufacturers. In this photosensitive polyimide, a photosensitive functional group is incorporated in the molecule of a polyimide precursor, and photoreaction occurs only in an exposed portion for polymerization, thereby varying the solubility between the unexposed portion and the exposed portion, and development is then conducted by making use of a solvent to dissolve and remove the unexposed portion while leaving only the exposed portion.
In subsequent heat treatment, the photosensitive groups have a poor heat resistance and are thermally decomposed and removed with the advance of cyclodehydration, so that only the polyimide portions having good heat resistance remain. The photosensitive groups can be introduced into the polyimide precursor through a covalent bond, an ionic bond or the like, and various photosensitive polyimides are commercially available.
The use of the above-described photosensitive polyimides, however, have the problem of high cost of the photosensitive polyimide per se, in addition to a large reduction in the film thickness which occurs as a result of the decomposition of a photosensitive group. Further, both non-photosensitive polyimides and photosensitive polyimides have the problem of poor moisture resistance.
For reference, the following patents may be mentioned as examples of patents describing processes for the formation of a pattern.
Japanese Unexamined Patent Publication (Kokai) No. 56-22428 discloses a process for the formation of a polyimide pattern, characterized by comprising the steps of (1) forming a film of a photosensitive polyimide precursor on a substrate; (2) irradiating the film with light in a pattern form and conducting development of the pattern; (3) heating the developed polyimide precursor pattern to convert the pattern to a polyimide pattern; and (4) treating the resultant polyimide pattern with an etchant for polyimide. A polyimide precursor bonded to a photosensitive group through a chemical bond or mixed with a photosensitive compound (such as bichromate) is used as the photosensitive polyimide precursor. In this process, the development residue of unexposed portions can be easily removed, so that through-holes free from defects can be obtained.
Japanese Unexamined Patent Publication (Kokai) No. 59-107346 discloses a heat-resistant photosensitive material comprising a photosensitive polyimide precursor including structural units represented by the following formula: ##STR1## wherein R.sub.1 and R.sub.2 are each an aromatic ring group, R.sub.3 is a thietane ring, and COOR.sub.3 is bonded to the amide group at its ortho-position. An insulating, heat-resistant polyimide pattern having good adhesion to the semiconductor substrate can be prepared through the use of such photosensitive material in the formation of a pattern.
In addition to these Japanese Kokais, although it is not considered to be relevant to the present invention, Japanese Unexamined Patent Publication (Kokai) No. 54-109828 discloses a heat-resistant photoresist composition which comprises 100 parts by weight of at least one polymer selected from a variety of organic polar solvent-soluble, heat-resistant polymers including polyimide, and 0.1 to 100 parts by weight, preferably 1 to 50 parts by weight of a monomeric compound containing at least two ethylenically unsaturated double bonds in its molecule. The amount of the monomeric compound incorporated into the heat-resistant polymer should not exceed 100 parts by weight, because larger amounts cause a reduction of the heat resistance of the resulting photoresist. This is because the specified monomeric compounds do not exhibit excellent heat resistance after curing of the resist.
The use of photosensitive polyimides as layer-insulating films in the production of integrated circuits wherein a considerable amount of heat is generated during use is desired in the art. As described above, photosensitive polyimides, however, have the problem of lowering the precision of the pattern due to the reduction in the film thickness resulting from the decomposition of the photosensitive groups during heat-treating of the photosensitive polyimide precursor to convert it into a polyimide, and further, photosensitive polyimides have the problem of the high cost of the material per se.
With regard to the use of the polyimide or polyimide resins, it is also well-known from Japanese Unexamined Patent Publication (Kokai) Nos. 62-30122, 63-86746 and 63-175854, for example, that the polyimide is mixed with curable resins or other additives such as polymaleimides, or novolak resins to form a polyimide resin composition useful in the formation of films or other molded articles having an excellent heat resistance and molding properties. However, the prior art publications including these Japanese Publications do not teach novel polymeric composite and production process thereof according to the present invention which will be described in detail hereinafter.
Further, hereinbefore, the polyimide resins including polyimide engineering plastics have been used in the formation of heat-resistant insulating layers, for example, protective films or interlayer insulators in the high-density mounting printed circuits, printed boards, electronic components and other devices. Because of lower dielectric constant thereof than that of conventional inorganic insulating materials, it is widely recognized that the polyimide resins are effective to improve characteristics of the integrated circuit (IC) devices, and in practice, they are able to ensure potential advantages such as passivation and interlayer insulation in microelectronics devices. Note that the polyimide insulators are considered to be more preferable than the inorganic insulators, because they generally have a dielectric constant of about 3.5 and also exhibit many excellent thermal and mechanical properties.
However, the polyimide resins, if used as the protective film or interlayer insulator in electronics and microelectronics devices including semiconductor devices, suffer from problems such as warpage of wafers and crack of chips. These defects are caused due to a large difference in a thermal coefficient of expansion (TCE) between the polyimide resins and the underlying materials such as aluminum as the wiring material or silicon dioxide (SiO.sub.2) as the passivation material, and can be varied depending upon the film thickness of the polyimide resins and the layer structure of the devices.
The defects due to the above-mentioned large difference in TCE can be reduced by using the polyimide resins having a low TCE as is disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 4-224824; U.S. Pat. No. 4,690,999; and Y. Misawa et al., IEEE Transactions on Electron Devices, ED-34, No. 3, March 1987. However, even if the low TCE resins are used, the defects cannot be avoided, when the polyimide resins are used at an increased thickness of film or the polyimide resins are subjected to the radiation exposure process.
In addition to the above drawbacks, recently developed photosensitive polyimide resins have problems that upon postbaking of the resins for obtaining the corresponding polyimide, a loss in the thickness of the resulting polyimide film can be increased to about 50% or more and also a stress of the film can be increased in comparison with the case using photoinsensitive polyimide resins, since photosensitive groups having a large molecular weight are volatilized from the film, along with water as a byproduct.